This invention relates to a flexible element for spectacle-frames which is apt to constitute a flexible bar for spectacles, having a high flexibility in two perpendicular planes and, in particular, in the plane which contains the longitudinal axes of the two bars of the frame and in the plane perpendicular to said plane.
There are known flexible bars comprising substantially a plurality of small blocks, between which there are disposed, alternately, elements provided with a spherical surface. The surfaces of each of said blocks which are facing said elements are spherical as well, and both the blocks and the elements are traversed by a core which is apt to hold them together. This allows to obtain a high flexibility of the bar in any plane passing through the axis of the bar and, in particular, also a certain rotation of the free end of the bar itself with respect to its end fixed to the frontal part of the frame; during the flexion of the bar there is a relative rotation of the blocks with respect to the spherical elements and, accordingly, a sliding between the coupled surfaces of said elements.
The above mentioned flexible bars have some disadvantages.
First of all, if it is desired the coupled surfaces of a block and of the respective spherical element to be very large, i.e., to have an area at least equal to or larger than that of the right section of the block, the diameter of said spherical elements must be very large. It follows that the element interposed between two contiguous blocks gives rise to rather large axial dimensions, measured in the direction of the longitudinal axis of the bar, and therefore, in a predetermined length of bar there can be housed only a rather small number of spherical elements and respective blocks coupled thereto.
Another disadvantage, which is more serious than the just mentioned one and which can be found especially in bars having a rather great height (i.e., in bars whose dimension measured in a direction perpendicular to the plane containing the longitudinal axes of the two bars of the frame is rather large), consists in that the radius of the spherical surfaces of the spherical elements which are necessary in this case, results in being too large to allow a good flexibility of the bar, in particular in the above-mentioned plane which contains the longitudinal axes of the two bars. In fact, in a ball joint, which the assembly formed by a spherical element and two contiguous blocks may be considered as comparable to, if the radius of the respective spherical surfaces is very large, for obtaining even a small relative rotation between two contiguous blocks it is necessary to have very great slidings between the coupled surfaces; obviously, great slidings hinder the flexion of the bar.